It is desirable for a flow meter to work while keeping a specified accuracy of measurement for extended periods, when a gas flow rate is measured for extended periods. However, a flow meter may be difficult to keep accurate measurement for extended periods, when gases to be measured contain various components or substances. Measurement of exhaust gas flow rate is described by taking that for an exhaust gas recirculation system for internal combustion engines as an example.
An exhaust gas recirculation (EGR) system recycles part of exhaust gases discharged from an internal combustion engine, e.g., vehicle engine, back to the intake side, to lower combustion temperature with gas components of low specific heat, e.g., CO2, and thereby to abate NOx emissions in the exhaust gases. In order to abate NOx emissions, it is necessary to accurately control the EGR rate. In a gasoline engine, inaccurately controlled EGR rate will cause troubles, e.g., difficulty in producing a desired torque or in controlling air/fuel ratio at a desired level. In a diesel engine, on the other hand, an excessive EGR rate will increase particulate molecule (PM) in the exhaust gases. Accordingly, high-accuracy EGR rate control is essential to abate NOx emissions while controlling evolution of PM.
A current EGR system, for example, adopts a control procedure which indirectly estimates EGR rate based on intake-side information, e.g., air flow sensor or throttle opening for estimating intake air rate. This procedure, although capable of controlling EGR rate in a steady-state condition, is incapable of accurately controlling the rate in a transitional operating mode. It is therefore desirable to accurately measure EGR rate in consideration of the exhaust gas regulations becoming more and more stringent both at home and abroad. Development of a gas flow meter which directly measures EGR rate is essential for accurately controlling the rate in an operational mode which needs the EGR control.
For air flow sensors which measure intake air flow rate, JP-A-59-104513 (Patent Document 1) discloses a basic structure. JP-A-2002-13960 (Patent Document 2) discloses an exhaust gas flow meter which uses a sensor heated to a temperature higher than gas flow temperature.
JP-A-2-114126 (Patent Document 3) discloses a sensor coated with a ZrO2-containing glass, JP-A-5-273020 (Patent Document 4) discloses a sensor coated with boron nitride as an outermost layer, and JP-A-2003-163105 (Patent Document 5) discloses a sensor coated with a mixture of TiO2 and glass serving as a protective coat.    (Patent Document 1) JP-A-59-104513    (Patent Document 2) JP-A-2002-13960    (Patent Document 3) JP-A-2-114126    (Patent Document 4) JP-A-5-273020    (Patent Document 5) JP-A-2003-163105